Filament supply system



Patented May 11, 1943 Winfield Kocli,l- Haddonfield; J., assigner toRadio Corporation of America, a corporation of Delaware ApplicationNovember Z2, 1940, Serial No. 366,583 12 claims. `(o1. '25e-17) Thisinvention relates to filament 'supplysystems and has for its 'principalobject the provision of -a method and means for improving the eiiiciencyof operation of' high power radici frequency amplifier tubes by varyingthe' nia-merit excitation in accordance with the modulating voltage.

The fila-ments of power tubes are ordinarily run at a' temperaturesufficient to provide the' necessary electron emission duringpeakmodulation. In ultra high power tubes' the current consumption requiredto so energize the filament or cathode is an appreciable portion' o'fthe operating cost of the transmitter. In addi'-` tion,- it is knownthat the useful life of the' tube is limited to a large extent by thellife of the cathode, and that the cathode life4 may be increasedsubstantially by reducing its average temperature.

' In accordance with the present invention,. the filaments of high powertubes are energizedwithy a current, preferably a radio frequencycurrent', modulated in accordance with the modulation voltage. Radiofrequency excitation aloney may be utilized, or in conjunction withadirect or low frequency `alternating current excitation which issufficient to maintain the filamentsat af low normal temperature, forexample, sufficient to provide satisfactory operation 'at carrier outiput or during periods of slight modulation.

In addition to the saving inv power duri-ng` standby periods, or duringperiods oflow modulation, a substantial increase inthe life ofthe tubeis' produced which of itself is an important factor. This is dueprincipally to the decreased` average emission of the filament, but`also due-to the more uniform current distribution of the filament whichis inherent in alternatingfrequency operation of the filament asdistinguished from direct current opration. It is kncivlnV that whendirect current is used, the filament tends to burn' out at one endbecause of .the non-uniforni electron emission. Another advantage ofradio frequency excitation of the filament is that troubles from 60'cycle hum are greatly reduced, or completely eliminated. Furtherobjectsof this invention therefore include the provision of an improvedfilament supply for high power tubes; the provision of means forincreasing the operating' efciency of high power transmitters; and

the provision of means for increasing the life of high power amplifiertubes' whilev at th'e' same time decreasing dific'ulties due tocommutator ripple; and hum which frequently arise from the' usef ofconventional filament excitation systems.

ThisI invention will be better understood' from the following'description when considered in connection withl the accompanyingdrawingjnin Wlifcl'il Figure' l i`s`- a schematic@diagramA of amutilated transmitter" in accordance wim this @toi invention, partly inblock diagram; Figure' 2 is a view of a tubular" filament; and 3v is aschematic diagram of a preferred form' of i'ri'` Vntio'n, also partly inblock diagram. Sriirlilar reference numerals refer A to 'similarelements through. the several figuresv of theY drawing.

In Fig'. 1, the output of a conventional crystal oscillator 5 iscoupledv to abuffern amplifier 1 which is', in turn; coupled through`a-l'iarnionic amplifier 9 and a modulated` amplifier'- Il to a poweramplifier I3. The oscillator 5' alsov supplies energy to a modulatedamplifier l5 the radio frequency' output of which is applied throughatran'sfo-rmer f1 to' the filament i9' o'f the output tube 2l of the`power amplifier I3. A microphone 23 is connected to a conventionalspeech amplifier 25, the 'output of which isY ap-` plied to themodulated amplifier |`5 through' a rectifier I6 and also through a` timedelay network- 21 to the modulated amplifier ll'.

While I have shown a power amplifier tube 2l having a filamentarycathode; itis to be understood that an indirectly heated' cathode mayalso'be utilized.- Preferably,- however; the filament takesthe form ofa'v tubular elementfoffthe type illustrated in Fig. 2. For exampl'ethefilament' may comprise atubular member 25J mounted at both ends bysuitablegmsupporting means 3| and 33 in the glass press 3,5, rIhetilarnent'is not necessarily hollow,` as illustrated,- although Ihavefound that its temperature'responds more rapidly to changes inexcitation ifa hollow member is used. p

, By Vusi-ng radio` frequency currentsvto energize the filament I amable to takeadvantage of a phenomena which `is well knownlto-thoseskilledin the art as askin effect. AThis effect is that radio frequencycurrents tend tol'travelnear the surface of a conductor, andconsequently radio frequencyV excitation tends toheat the'outer surfaceof the filament more rapidly than 4the interior of the filament. Forexample, the skin thickness constant of copper at onemegacycle isy.approximately` .006 centimeter. LThis means that a `radio frequencycurrent of one megacycle flowing th'rough` acopper conductorwilltflowsubstantially through the' cond-uctort a depth .of only' .o'xcenti'meterfrom the outersurface. The 'skin thickness constant increases'inverselyas the square root ofthe frequency`-`-l It is appreciated that afilament has a certain amountV of thermal inertia, that is, the mass ofthe filament tends to heat slowlsrr upon the application of a urrent. Aconventional` filament energized by direct or low frequency alternatingcurrent, would'. therefore, respond tooslowlyjt'o a varyingcurrentcontrolledL by the modulation voltage tol havefanyv appreciableeffectincreasing the" cathode emission-,during peaksjin'themodulatingvoltage. However', by'employing radio fre"- quency currents,and possibly a hollow filament, the thermal inertia is sufficientlyreduced to make practical the increase of emission in response tochanges in the modulating voltage since the temperature of only a smallportion of the mass of the filament is affected.

Upon the application of sound waves to the microphone 23 a modulatingvoltage is applied to the modulated amplifler I5, the effect of which isto increase the normal output of the amplifier above the minimum valuenecessary to maintain the filament I9 at, say, an incandescent state.The rectifier IB is included in the circuit between the microphone andthe modulated amplifier I5, in order to remove the negative half of themodulation voltage. The increased output of the amplifier increases thefilament emission an amount sumcient to increase the available electronsso as to increase the current capacity of the power amplifier tube 2|.At the same time, or at a slightly subsequent time when the time delaynetwork is used, the modulating voltage is applied to the modulatedamplifier II to increase the potential of the power amplifier grid andupwardly modulate the carrier current in the conventional manner. Byincluding a time delay network 2'I in the circuit between the speechamplier 25 and the modulated amplifier II a short period of time isprovided in which the cathode may reach peak excitation so that the tubewill be ready to carry its maximum current by the time its excitationfrom the modulated amplifier Il is applied.

In Fig. 1 the entire filament excitation is provided by the radiofrequency current derived from the modulated amplifier I5. In accordancewith a preferred embodiment of my invention, however, conventionalexcitation methods are employed to supply a minimum current to maintainthe filament at a temperature sufficient to operate the tube duringperiods of low modulation corresponding to the carrier output condition.Such a system is illustrated in Fig. 3 to which reference is now made.

The essential elements of the transmitter from the crystal oscillator 5to the power amplifier I3 are the same as those described in connectionwith Fig. 1. The modulated amplifier I5 is supplied with radio frequencyenergy from the oscillator 5, as before, and its output is coupledthrough a transformer I'I to the filament I9 of the power amplifier tube2|. In this case, however, blocking condensers 31 and 39 are connectedin this circuit and the filament is also energized by a parallelconnection, through a pair of radio frequency choke coils 4I and 43 by atransformer 45 connected to the 60 cycle power line. In this case, thestandby or low modulation filament excitation current is supplied atmaximum efficiency to the filament from the conventional power line.During modulation peaks, however, the excitation is increased by a,superimposed radio frequency current derived from the modulatedamplifier I5. Since the minimum excitation is determined independentlyof the amplitude of the radio frequency energy, a rectifier is notrequired in the modulation control circuit.

Radio frequency excitation is applicable not only to the power amplifiertube, but also to other tubes of the transmitter, although it will beappreciated that the greatest saving will be obtained where the systemis used in high power tubes in which the filament current isconsiderable. Thus in a high level modulation system of the typeillustrated in Fig. 3 radio frequency energizing currents, alone, or incombination with low frequency or direct current excitation, may beapplied to the high level modulator 41. In the case illustrated, theanode-cathode path of the modulator tube 49 is connected in the groundreturn of the cathode of the power amplifier tube 2|, the impedance ofthe tube thus constituting a biasing resistor for the output amplifier.The filament 5I of the modulator tube is coupled to a winding 53 of theradio frequency output transformer I'I, this circuit also includingisolating capacitors. The grid of the modulator tube 49 is coupled to acarrier speech amplifier 55 through a time delay network 21.

Radio frequency excitation of the filament may also be applied to lowpower tubes to provide a regenerative feedback circuit which increasesthe operating speed of the system. Thus the filament of the modulatedamplifier l5 is coupled to the output transformer I'I as is the filamentof the speech amplifier 25. As a result, a modulating voltage whichtends to increase the excitation of the power amplifier and modulatorfilaments likewise increases the excitation of the modulated amplifierI5 and the speech amplifier 25. As a result, the radio frequency outputof the modulated amplier I 5 tends to build up more rapidly in responseto an increase of the modulating voltage than it would in the absence ofthe feedback. Tube saturation, however, prevents the system from goinginto oscillation by reason of the regenerative feedback coupling throughthe filament.

The frequency of the radio frequency current used to energize thefilaments of the tubes is preferably a different frequency from thatamplified by the output tube. In the case illustrated, for example, thisis accomplished by including a harmonic amplifier between the oscillatorand the output. Thus the filament excitation frequency is a sub-harmonicof the output frequency. The nlament excitation frequency should not,however, be selected at random since beat frequencies would then beproduced in the output tube. I have thus described a system forsubstantially reducing the power required to energize the filaments ofhigh power radio frequency amplifier tubes by reducing the excitationduring standby periods or periods of low modulation. In addition, thesystem increases the life of the tubes and reduces hum and commutatorripple in the system.

I claim as my invention:

l. In a modulated transmitter including a thermionic discharge devicehaving a cathode electrode, the method of operation which includesproducing radio frequency currents in said device, modulating saidcurrents, utilizing other radio frequency currents to energize thecathode of said device, and varying the amplitude of said other currentsas a function of said modulations to produce a substantial variation inthe energization of said cathode corresponding to said modulations.

2. In a modulated transmitter including a discharge device having acathode and a grid electrode, the method of operation which includes thesteps of applying first heating currents to said cathode, superimposingradio frequency heating currents on said cathode, applying a modulatingvoltage to said grid. and varying the amplitude of said radio frequencyheating currents in accordance with said modulating voltage to produce asubstantial change in the heating of said cathode corresponding to saidmodulating voltage.

3. In a system including a thermionic discharge device having a hotcathode electrode, the method of operation which includes utilizing saiddevice to produce modulated radio frequency signals, generating radiofrequency currents bearing a fixed harmonic relation to the carrierfrequency of said signals, applying said currents to said cathode toproduce a resultant heating thereof, and varying the amplitude of saidcurrents as a function of said modulations to produce a substantialvariation in the electron emission from said cathode corresponding tosaid modulations.

4. In a system including a thermionic discharge device for amplifyingmodulated radio frequency signals, said device having input and outputcircuits and a thermionic cathode electrode, the method of operationwhich includes the steps of applying modulated radio frequency signalsto said input circuit, applying rst heating currents to said cathode,generating other radio frequency currents bearing a fixed harmonicrelation to the carrier frequency of said modulated radio frequencysignals, superimposing said radio frequency currents on said cathode toincrease substantially the excitation thereof, and varying the amplitudeof said radio frequency currents in accordance with the modulations ofsaid signals t produce a resultant variation in the excitation of saidcathode.

5. In a system including a thermionic discharge device for amplifyingmodulated radio frequency signals, said device having a cathodeelectrode, the method of operation which includes the steps of heatingsubstantially only the outer surface of said cathode to energize saidcathode, and varying the intensity of said heating in accordance withthe modulations of said signals.`

6. In a system including a thermionic discharge device for amplifyingmodulated radio frequency signals, said device having an input circuitand a thermionic cathode electrode, the method of operation whichincludes the steps of applying modulated radio frequency signals to saidinput circuit, applying a first heating current to said cathode toproduce only sufficient electrons for normal signal levels, generating asecond heating current for said cathode, the frequency of said secondheating current being such that substantially all of said current newsalong the surface of said cathode, and Varying the intensity of saidsecond heating current to increase the electron emission of said cathodeduring upward modulations of said signals above said normal signallevels.

'7. In a transmitter comprising a power amplier output tube, a modulatortube, means for applying a radio frequency voltage to said output tube,and means for applying a modulating voltage to said modulator tube, saidtubes having thermionic cathode electrodes, the method of operationwhich includes the steps of applying first heating currents of constantaverage amplitude to energize said cathode electrodes, generating othercurrents of radio frequency, applying said other radio frequencycurrents to the cathode electrode of at least one of said tubes toincrease the heating thereof, and

controlling the amplitude of said other radio frequency current inaccordance with said modulating voltage to thereby substantiallyincrease the heating of said cathode during periods of maximummodulation.

8. A modulated transmitter comprising the combination of an outputamplifier tube having a thermionic cathode, means for applying modulatedradio frequency currents to said amplifier, means for generating otherradio frequency currents, means for applying said other currents to saidcathode to produce electron emission, and means for varying theamplitude of said other currents in accordance with said modulations toprovide a substantial corresponding variation in the electron emissionfrom said cathode.

9. In a modulated transmitter, the combination of an amplifier tube forradio frequency currents, said tube having a thermionic cathode, meansfor applying rst heating currents to said cathode, efficient radiofrequency transfer means for applying auxiliaryradio frequency currentsto said cathode to increase the excitation of substantially only theouter surface of said cathode, and means for varying the amplitude ofsaid auxiliary currents and said radio frequency currents in accordancewith modulating signals.

l0. In a modulated transmitter, the combination of an amplifier tubehaving a thermionic cathode, means for applying signal-modulated radiofrequency currents to the input circuits of said tube, first means forenergizing said cathode to produce an electron emission just sufficientto produce normal output in the absence of modulation, and meansincluding a source of radio frequency current modulated by the samesignal for increasing the cathode excitation during periods of upwardmodulation to increase the electron emission of said cathode and toproduce peak output during said periods.

11. In a modulated transmitter including an amplifier having input andoutput circuits and an electron-emissive cathode electrode, the methodof operation Which includes applying modulated radio frequency inputcurrents to said input circuit, deriving modulated radio frequencyoutput currents from said output circuit, and applying other similarlymodulated and amplified radio frequency currents to said cathodeelectrode, the amplitude of said other radio frequency currents beingsuicient to produce a substantial variation of the cathode to Vary itselectronemission in accordance with the modulations of said radiofrequency output current.

12. In a modulated transmitter including a discharge device having acathode and grid electrode, the method of operation which includes thesteps of applying modulated radio frequency currents to said gridelectrode, applying first heating currents to said cathode,superimposing radio frequency heating currents on said cathode, varyingthe amplitude of said radio frequency heating currents in accordancewith the modulation envelope of said radio frequency currents, andadjusting the amplitude of said first heating currents to limit theelectron emission of said cathode to a Value sufficient only to carryoutput currents of carrier level.

WINFIELD R. KOCH.

